Load handling hoist systems which utilize electric motors to raise and lower a load are used in a wide variety of industrial applications and, consequently, they are operated often under circumstances which are extremely hostile to longevity of the hoist systems. For this and other reasons, such hoists are normally provided with safety switches such as a limit switch which is mechanically actuated when the hoist hook or load engaging hook reaches substantially the upper limit of its travel. Operation of the switch, if it occurs, prevents the hoist operator from raising the load beyond the limit and thereby is intended to prevent damage to the hoist. Such electro-mechanical devices however are propense to maladjustment or failure.
Another problem which faces operators of industrial hoists is encountered when a large workpiece is to be actively positioned with respect to a piece of equipment such as a lathe or other metal forming tool.
In such circumstances, it is desirable to lower the load gently from an elevated, suspended position to a final position. This type of maneuver has proven difficult to accomplish. In an attempt to solve this positioning problem, a variety of approaches have been used as for example in the following U.S. Pat. Nos.: 3,730,484, 2,752,120, 2,801,760.
To solve the problem associated with "jogging" the load to final position a precise load positioner is disclosed in U.S. Pat. No. 4,361,312 of Nov. 30, 1982, assigned in common herewith. This patent is directed to apparatus which may be retrofit in a single speed or dual speed hoists, the device being provided to modify a standard hoist in order to enable that hoist to position a suspended load very precisely. The patented device operates by temporarily causing the hoist brake to be released for a predetermined limited period without actuating the hoist motor so that the load is lowered through a small incremental distance under the influence of its own weight.
Inter alia, the aforesaid commonly assigned patent discloses a system in which a gear tooth detector is utilized to monitor the pinion gear of the hoist motor to generate a digital pulse corresponding to the passage of each of the teeth of the gear as it passes the location of the transducer. The pulse signal is then delivered to a digital counter which has been preset to count the pulses from the transducer when the brake release lowering operation is effected. When the total number of pulses thus received from the transducer equals the predetermined number of pulses selected by the operator and dialed into the digital counter/comparator, the counter/comparator terminates the brake release control signal thereby allowing the brake to reengage so as to halt load descent. Thus, in this particular embodiment of the patent, the control signal from the precise load positioner is dependent upon load movement rather than upon time, other embodiments of the patented device being effective to interrupt the brake release control signal after a predetermined period of time.
Other patents of which applicants were aware at the time of filing this application are as follows:
______________________________________ Santini et al 2,403,125 Crookston 2,656,027 Bogle 2,752,120 Logan 2,912,224 Buck 3,053,344 Ancheta 3,883,859 Joraku et al 4,087,078 Australian 283,230 (10/1965) U.K. 826133 (12/1959) ______________________________________